The present invention relates to a method of indicating a basic air-fuel ratio condition of an internal combustion engine which has an air-fuel ratio feedback control system.
There presently exists an engine equipped with an air-fuel ratio feedback control system which controls an air-fuel ratio of the mixture supplied to the engine in response to an air-fuel ratio correction signal. The air-fuel ratio correction signal is gradually increased or decreased in accordance with an output of an exhaust gas sensor which detects the concentration of a particular component contained in the exhaust gas.
In such an engine, a basic air-fuel ratio, which is equivalent to an air-fuel ratio of the engine when the air-fuel ratio feedback control operation is stopped, is adjusted by detecting the voltage level or the digital value of the air-fuel ratio correction signal, and by setting a mixture control mechanism, for example a by-pass air adjusting screw of an air-flow sensor, so that the voltage or digital value of the air-fuel ratio correction signal approaches a predetermined value.
If the engine employs an analog type air-fuel ratio feedback control system, the adjustment of the basic air-fuel ratio is carried out by setting the by-pass air adjusting screw of the air-flow sensor so that the voltage of the air-fuel ratio correction signal approaches a predetermined voltage level. Therefore, at the adjustment of the basic air-fuel ratio of the engine having an analog controlled air-fuel ratio feedback system, it is necessary to use a voltmeter or another voltage measuring device for measuring the voltage level of the air-fuel ratio correction signal.
On the other hand, in an engine employing a digital controlled air-fuel ratio feedback system using a microcomputer, the air-fuel ratio correction signal is binary coded and is not measured by a voltmeter. Therefore, in the engine having the digital controlled air-fuel ratio feedback system, the binary coded signal stored in the microcomputer must be taken out and must be converted into a d-c voltage using a digital-analog converter (D/A converter), to adjust the basic air-fuel ratio, thus requiring a very expensive and complicated device, in addition to the voltmeter.